The 27mm is a very nice lens and equivalent to a "perfect normal" 40mm (or 41). I usually put the 35 on my newly acquired X-Pro, but if I were to have only one lens, I'd seriously consider the 27. Many old fixed lens rangefinders by Olympus (which I owned) and other manufacturers chose the 40mm length for their cameras. Sigmas DP2 Merrill has roughly a 42mm equivalent. If you only have one lens, it is a very nice choice, and it makes the X-Pro a pretty nifty small package. I wouldn't worry about no aperture ring: the rear dial to change f-stop is right there, easily under your thumb. I've never had to hunt for it with the camera to my eye.
That said, it would be a shame to have the system and miss out on the 35mm, which is a special lens. Once sharp across the frame at f4, it has no competition.
As for the X-Pro itself. I've wanted its hybrid viewfinder for ages, so even owning the E-M5, which I like a lot and intend to keep, I'm thrilled with this camera.